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Pressure "spikes" on Big Magnum Cartridges....

 
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  #1  
Old 10-12-2006, 01:25 AM
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Pressure \"spikes\" on Big Magnum Cartridges....

Some of you guys shoot some big capacity cartridges. The weather is turning cooler here and I've kind of semi retired and have some time to try and work with my 30-378 I bought years ago but hadn't shot alot.

HERE'S THE SITUATION....I'm at the range the other day finishing off a 6 mo. old batch of 200 accubonds over 118 grains bmg. My notes when I shot the freshly loaded batch said "shoot great" "velocity 3170. This in a stock sako with 26" barrel. Well today I shoot about 8 rounds and they are clocking about 3250....and I get one with a sticky bolt and one more with a realy sticky bolt. The sun was just getting high enough over my chrono where I didn't get readings on the sticky bolt shots.

I stop there and go home with my last 3 rounds still loaded.
I can't figure out what's going on. Maybe I missed trimming these last ones....no they check out ok. Then I get to thinking that in the past I had noticed that as ammo gets old the bullets sometime take a sort of "set" to the case. I thought I had cured this by swabbing out lube with damp paper towels wrapped around a small nylon bore brush.
Anyhow I take the last three rounds and put them in my seating die and try to seat them deeper and sure enough....there is somewhat of a "crack" as they let loose.

Have you guys ever experienced this kind of "bullet set" or bonding (if you will) on stored ammo? Am I crazy to think this can have any more effect than....say.....crimping?
Is it possible my gun was just "fouling out" and that caused excessive pressure to come on that fast???

ANY OTHER IDEAS PLEASE????????

BTW--I checked a 6 year old batch of xlc barnes loadings and the coating sure keeps them from taking any kind of set.
They seated deeper as if I was just loading them tonight.

Maybe a couple of drops of flitz in my tumbling meadia is putting a coating on my brass that is "unfriendly" to storage of regular jacketed bullets?
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  #2  
Old 10-12-2006, 12:13 PM
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Join Date: Jul 2004
Location: Texas
Posts: 5,438
Re: Pressure \"spikes\" on Big Magnum Cartridges....

Kraky

First of all you did the wright thing by stoping.

If all of those "old" loads were loaded at the same
time and with the same batch of powder/bullets/primers
I think the problem was heat.

Older powders become heat sensitive (more volatile).
But 6 months is not old.

While competing in a 1000 yrd match I had the same
thing happen to me.

Started shooting and everthing was going very good
but soon I started chaseing the bullet strike and ended
up taking over 2&1/2 moa off before the problem showed
its ugly head by blowing a primer.

being a little slow on the uptake I proceeded to blow two more primers then I stoped .

Laying there in the hot sun trying to figure out what was
wrong I had an epiphany(brain fart)my ammo was in the sun
and felt hot to the touch.

So I laid there and used up about half of my time waiting
for the ammo to cool down so I could finish the match.

Sure enough after cooling it shot fine.

Note: These loads were MAX and the heat only made them
More so.

Your loads were progressively getting hotter thats why
I think this was the problem.

Sounds like you are very particular in your reloading so
heat would be one of the variables.

Good shooting
J E CUSTOM
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  #3  
Old 10-12-2006, 03:49 PM
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Join Date: Sep 2001
Location: medford wi usa
Posts: 273
Re: Pressure \"spikes\" on Big Magnum Cartridges....

Thanks for taking a shot at my problem but unfortunately we shoot out of nice, cool, shady, shooting shed and the ammo wasn't in the sun. I even found a record where I had loaded the exact same load a year earlier with no pressure problems (with the same lot of powder). I do trust my scale (rcbs1010) and I always double check the scale for the first few rounds against a digital scale so I know it was on the money. Heck with that big of a case even a grain more powder would only result in about 20-30fps.
I did run a boresnake down the barrel before switching to this batch of ammo. Maybe I stirred something up in there that changed the friction inside the barrel and a near max load became nasty. I'd appreciate any other guesses but I am thinking somehow I got a spike of at least about 10k on the one shell. The primer didn't fall out but I'm thinking it had to be darn close to it and with that big heavy case I had to be way over max.
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  #4  
Old 10-13-2006, 02:34 AM
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Join Date: Dec 2002
Location: mathias wv
Posts: 985
Re: Pressure \"spikes\" on Big Magnum Cartridges....

The barrel heating up will warm the ammo enough to do this, I've saw the same thing in my 7mm AM, leaving the third round in the chamber an extra 2 min. will boost my MV over 200 fps. but I shoot WC872 and its pretty heat sensative, on an 80 degree sunny day, a typical 3 shot string goes like this first shot is fine, light bolt lift, effortless extraction, shot #2, slightly heavy lift, slightly sticky extraction, mildly cratered primer, third shot, !@$%*& heavy bolt lift, and very sticky, primer may or may not pierce. 60 degrees and below there isn't a problem.
JS
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  #5  
Old 10-13-2006, 10:22 AM
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Location: medford wi usa
Posts: 273
Re: Pressure \"spikes\" on Big Magnum Cartridges....

I can see how that can happen with a warm barrel--much like leaving the ammo in the sun. It was about 45 degrees this day and I was shooting by loading one at a time so the bullet was only in the barrel about 15 seconds. I'm going to back the loads down about 2.5 grains and see where "they clock". I'm expecting to lose only 50-60fps and hopefully get in a "zone" where I have a little lee-way for "crap happens".
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  #6  
Old 10-13-2006, 12:09 PM
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Join Date: Jul 2004
Location: Texas
Posts: 5,438
Re: Pressure \"spikes\" on Big Magnum Cartridges....

Kraky

I load 116gr of 50 bmg in mine and its healthy so 118 could
be on the edge.

50 bmg is very forgiving and does fowl but 8 ronds shold
be no problem.

You might try a real good cleaning and check freebore.

When you find the problem please let the site know.

It may save someone else from going through what you are.

Good luck
J E CUSTOM
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  #7  
Old 10-13-2006, 12:41 PM
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Join Date: Sep 2001
Location: medford wi usa
Posts: 273
Re: Pressure \"spikes\" on Big Magnum Cartridges....

JE----THANKS for the input. Does anyone care to share some successful loads with 165-200 grainers with the 30-378?

Here is a sample pack I intend to load up this afternoon and get out and shoot soon. They are repeats of past loads or some new ones I'm speculating on.

104 retumbo under 200 accubond
101 Re 25 under 200 accubond
105 Ramshot mag under 180 partition
121 AA8700 under 180 partition
109.5 H1000 under 180 accubond

Maybe when I'm doing this type of testing (about 5-6 rounds of ea load) the mix of powders etc are fouling me out although the gun shoots good all the way through the test. My gun is one of the cheap sako TRG-s that was on closeout lists about 6 years ago for about $600. It's been a decent shooter.....not stellar...but decent. From what people say it has a terrible recoil lug that DOES NOT help the accuracy picture to much.
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